| Due to the diversity in geometry and composition, perovskite oxides possess plenty of special physical properties, such as colossal magnetoresistance, catalytic activity, ferroelectricity, large dielectric constant, oxygen conductivity, and high-temperature superconductivity. These properties make perovskite oxides promising for extensive potential applications as magnetic memory, magnetic sensor, photocatalyst, and solid oxide fuel cells, etc., and become the focus of a large number of experimental and theoretical studies.Spintronics is currently a hot research field in which half-metal materials play a key role, and perovskite oxides are the potential compounds with half-metalicity. Hence, computationial search for half-metallic provskite oxides is the main motivation of this work.In chapter1, an overview is presented on the research progress of provskite oxides as colossal magnetoresistance materials. In the mean time, a brief review is provided on spintronics.In chapter2, density functional theory (DFT) is introduced and a review is given on its development in recent years, including the basic framework of DFT, good approximation of exchange-correlation function, and self-interaction correction etc.. At the end of this chapter, we briefly introduce some simulation packages based on DFT.In chapter3, the special properties of SrFe1-xCoxO3are studied by using DFT. The results show that the ground states have A-type antiferromagnetic order for x=0.1and ferromagnetic order for x≥0.2with Co ions distributed averagely. SrFe1-xCoxO3exhibits half-metallic nature for0.2≤x≤0.7and full-metallic nature for other values of x, and the half-metallic gap decreases with increasing x. The tunneling between the half-metallic ferromagnetic phases leads to the large magnetoresistance. In addition, the Co cations are in the intermediate-spin state, while the Fe cations are in the intermediate-spin state for x≤0.5and the high-spin state for x≥0.6.Based on the results of the chapter3that SrFeo.5Co0.5O3is half-metal and considering the oxygen vacancy in the system, new half-metallic materials, non-stoichiometry peroskite oxide Sr2FeCoO6-δ (5=0,1, and2), are designed in chapter4. The results reveal that the ground-state Sr2FeCoO5has antiferromagnetic half-metallic nature with Imma space group. The magnetic calculation illuminates that Sr2FeCoO5exhibits magnetic moment ordering with the magnetic moments of2.91and3.72μB on Fe(1) and Fe(2) sites,2.96and3.20μBb on Co(1) and Co(2) sites antiparallel to those of Fe ions, respectively. The magnetic moments of about0.05μB per atom parallel to those of Co ions are induced on O sites. The net magnetic moment per unit is zero. Additionally, Sr2FeCoO4behaves as ferrimagnetic half-metal, and Sr2FeCoO6exhibits ferromagnetic half-metal, respectively. This hints that Sr2FeCoO6-δ possesses half-metallic nature in a large range of δ. It is expected that the present findings may be a way to develop a new kind of half-metallic material.In chapter5, the origin of the intriguing physical properties of an A-site-ordered LaCu3Fe4O12perovskite is investigated in detail. The calculational results show that Coloumb absorption force between Fe-O-Fe at high temperature is stronger than that at low temperature, which is the origin of the large negative thermal expansion in the compound. The origin of the stability of the Cu3+oxide, LaCu3Fe4O12, is the strong hybridization interaction between La-O-Cu, which also suppressed the structural change with the temperature changing. The equivalence symmetry of each Fe ion at the same temperature results into the charge transfer between Fe and Cu, instead of between Fe and Fe. We think the special properties of LaCu3Fe4O12are driven by the strong hybridization interaction between La—O—Cu. Therefore, whether there is the hybridization interaction between A-O-A’would essentially dominate the properties of A’A3B4O12, and the different characterful material satisfying the different technologically applications could be experimentally designed by changing the A-site component. |
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